(1) Field of the Invention
The present invention relates to a plasma processing apparatus and a plasma processing method and, more particularly, to a plasma processing apparatus and a plasma processing method that make it possible to restrain influences attributable to fluctuations in process performance, which occur as plasma processing proceeds.
(2) Description of Related Art
As a plasma processing apparatus, there has been known, for example, a plasma etching apparatus adapted to introduce an etching gas into a vacuum processing chamber to generate plasma under reduced pressure and to cause radicals or ions generated by the plasma to react with a wafer surface, which is an object to be processed, thereby accomplishing plasma etching.
In the aforesaid plasma processing apparatus, with scaling down of semiconductor devices, it has been becoming difficult to obtain desired processing performance (hereinafter referred to as “process performance”) each time due to various disturbances even if the processing is carried out under the same manufacturing condition (hereinafter referred to as “recipe”).
Accordingly, as a solution thereto, an advanced process control technique (hereinafter referred to as “APC”), which is capable of restraining the influences of disturbances, has been adopted. The APC is a method, in which, normally, plasma processing to be carried out according to a preset recipe is performed while conducting feedback control or feedforward control thereby to dynamically correct the recipe.
In particular, a control method for restraining fluctuations in process performance by correcting a recipe, which is a processing condition, for each processing cycle, such as for each wafer or each lot, is also reputed to be “Run-to-Run control” (hereinafter referred to as “Run-to-Run control”). A recipe includes a plurality of processing condition items (hereinafter referred to as “parameters”), such as a radio-frequency power, a gas flow rate, a pressure, and processing time. One or more parameters among these parameters are corrected by the Run-to-Run control.
For example, a plasma etching apparatus normally carries out plasma etching according to the processing conditions specified by a recipe. However, the process performance results (e.g., etching rate, processing dimensions or the like in the case of a plasma etching apparatus) inconveniently fluctuate with time in some cases due to substances adhering to the inner wall of a plasma etching chamber, or changes in the wear on components in the plasma etching chamber. In order to reduce or prevent the aforesaid time-dependent fluctuations, the Run-to-Run control for correcting a recipe for each processing cycle is applied.
As an example of the Run-to-Run control, JP-A-2003-017471 discloses a plasma processing apparatus carrying out processing on a workpiece placed in a vacuum processing chamber, and that plasma processing is controlled according to a recipe generated by an optimum recipe calculation model on the basis of sensors that monitor processing amounts during processing and the estimation results of a processing result estimation model.
Further, JP-A-2003-068717 discloses a semiconductor processing apparatus which includes a sensor monitoring the processing condition of the semiconductor processing apparatus which processes a semiconductor wafer, a processing result input unit which inputs measured values of a processing result of the semiconductor wafer processed by the semiconductor processing apparatus, a model formula generating unit which generates a model formula for predicting a processing result on the basis of sensor data obtained by the sensor and the measured values by using the sensor data as an explanatory variable, a processing result predicting unit which predicts a processing result on the basis of the model formula and the sensor data, and a processing condition controlling unit which compares the predicted processing result with a set value established beforehand and controls a processing condition of the semiconductor processing apparatus so as to correct the difference therebetween.
In general, when a process performance result begins to deviate from a desired value, the Run-to-Run control functions to return the deviating result to the desired value. According to the method based on the Run-to-Run control, a correction amount of a parameter of a subject recipe is calculated on the basis of the difference between a process monitor value correlating with a process performance result and a target value of a process monitor value determined beforehand from a process performance result, and the corrected recipe is applied to the next plasma processing. Generally, however, a parameter of a recipe has an upper limit value and a lower limit value that can be corrected.
In the Run-to-Run control, carrying out plasma processing according to a particular recipe that has a parameter exceeding a predetermined range may adversely affect a process performance result. For example, in plasma etching, setting a radio-frequency bias power at a value that exceeds a predetermined value when controlling an etching depth by means of a radio-frequency bias power to be applied to a wafer may damage a mask member, leading to a problem, such as a failure to obtain a desired shape.
Meanwhile, the aforesaid parameter control range is subjected not only to process performance restrictions but also to hardware restrictions. For instance, in the case of a mass flow controller for controlling a gas flow rate, the control range will be a range of about 5% to about 100% of each full scale (maximum flow rate). This means that a gas flow rate exceeding the range cannot be controlled.
Accordingly, in the Run-to-Run control, a correction amount of a recipe is provided with an upper limit and a lower limit so as not to carry out the control beyond the range defined thereby. Normally, if a correction amount of a recipe deviates from the range, it is determined as a control error and plasma processing is interrupted. However, such the interruption of the plasma processing results in a significantly reduced operating rate of a plasma processing apparatus. For this reason, there has been a demand for Run-to-Run control that can reduce or restrain the interruption of plasma processing.